Laser printers, laser based rapid prototyping equipment and laser (micro) machining centers and the like make use of a precision focussed laser beam that is scanned across a flat scan surface along a straight line path. A typical optical scanning system for this purpose employs a rotating or oscillating flat mirror to deflect the incoming (collimated) optical beam. The (collimated) beam is aimed at the scanner mirror's rotational axis, so that the deflected beam sweeps a plane in space. The beam thus strikes the flat scan surface in a spot that scans along a straight line path. In order to focus the scanning beam onto the flat scan surface, and to ensure that the focussed spot location has a linear relation to the mirror deflection angle, the deflected beam passes a so called f-theta objective. The f-theta objective can be a (multi) lens element system or a catadioptric system comprising both lenses and mirror elements.
Wafer defect and document scanners also incorporate a scanning element and an f-theta objective, either these systems are used to relay the light, provided by an illuminating source, reflected (or transmitted) by a small spot on the object of interest through the f-theta lens and the scanning element towards a stationary photodetector. The signal generated by the photodetector provides information on the status of the object at that specific observed spot location. When the scanner is operating the spot location under observance is moving versus the objective. This type of scanners is used to provide data at very high resolving power (>10000 pixels per line), where (line) cameras are not providing the performance required.
In some systems (so called co-axial) systems both the illuminating beam and the reflected light beam pass through the scanner system. A dedicated beam splitter at the conjugate side of the scanned path splits the illuminating and reflected light beams.
The design of f-theta objectives is straightforward and many designs are commercially available. For laser material processing a typical f-theta objective has a focal length of 80 to 256 mm. These designs achieve reasonable performance for (optical) beam deflection angles of up to about 18 degrees. For laser beam scanners, the minimal focussed spot size that can be achieved is determined by the diameter of the input collimated beam and the focal length of the f-theta objective. The length of the scanned path is then determined by the same focal length and the maximum deflection angle the objective can handle. Typical operating parameters for a commercially available system are for example beam diameter 10 mm, laser wavelength 532 nm, focal length of the objective 160 mm, length of the scanned path 110 mm, and focussed spot TEM00 diameter of 17 μm.
Commercially available compact f-theta lenses as described above have some major drawbacks when used for laser material processing. Laser material processing is often of ablative nature and require a threshold energy density level (J/cm2) in the laser beam before any processing is starting. For most applications the threshold energy density level is to be kept constant across the scanned path within very narrow margins. With compact f-theta lenses the radiation-material interaction changes considerably across the scanned path: in the middle the focussed spot is circular, but at the extremities of the scanned path the spot shape is elliptical. This is caused by the fact that the beam impinges on the flat scan surface in non-perpendicular conditions. As said before, the maximum deflection angle is usually about 18 degrees, which causes the spot to grow by about 5% at the extremities of the scanned path. Since the total beam energy is the same in the middle and at the extremities of the scanned path, the peak energy density of the beam will drop at the extremities of the scanned path, because of the larger beam size. Inevitably this will be seen on the processed surface.
To overcome this negative effect, a telecentric f-theta objective may be used. In a telecentric objective, the chief ray of the focussed laser beam is always perpendicular to the flat scan surface, across the complete scan. Telecentric f-theta objectives are for example described in U.S. Pat. No. 4,863,250 and U.S. Pat. No. 4,880,299 and are commercially available, but unfortunately they are much more expensive than standard f-theta objectives since they usually have more optical elements and the last optical element must be larger in diameter than the scanned path. This is the main reason why those commercially available telecentric f-theta objectives are limited to a 50 mm scanned path.
To lower the cost, telecentric systems using at least one mirror surface have been designed. Mirrors have a cost advantage over lens elements above a certain element size, e.g. in case lens elements would become larger than 100 mm. Such telecentric systems have for example been described in U.S. Pat. Nos. 5,168,386. 5,168,386 describes, as illustrated in FIG. 1, a flat field telecentric scanner including a planar scan deflector 91 moveable about a scan axis 93 and placed in the path of an incident light beam 95a. Two off-axis mirrors 97, 99 in series in the path of the scanning light beam 95b reflected from scan deflector 91 produce a telecentric scan of the light beam 95c incident on a target surface in an image plane 101. Mirror 97 is a weak spherical convex mirror, and mirror 99 is a spherical concave mirror. A basic characteristic of this design is that most of the focusing action is taken up by a lens (or other focusing system) in front of a scan deflector 91, and that the post deflector optics 97, 99 function as a field flattener. Besides scanning and field flattening the optical system must also focus the beam to a sufficiently small, preferably diffraction limited spot. This means that the optical aberrations in the overall optic design must be kept low. Optical systems having wave front aberrations lower than a quarter wave, or having a calculated ray traced spot size smaller than the Airy spot, are usually considered diffraction limited. The embodiment disclosed, with two spherical mirrors in series in the path of the scanning light beam 95a, 95b, 95c is indicated not to have enough degrees of freedom to produce a flat-field telecentric scanner where all aberrations are controlled. In particular, astigmatism (i.e. rays of light beams propagating in two perpendicular planes having different foci) cannot be made negligible in this simple system.
This is illustrated in FIG. 2 to FIG. 5. FIG. 2 shows a side view of a telecentric system using spherical mirror surfaces as in FIG. 1, while FIG. 3 shows a front view thereof. System parameters of the system illustrated are focal length=190 mm, scan width=170 mm, diffraction limited spot 1/e2 diameter 14 μm at 354 nm wavelength. In the illustration of FIG. 3, light paths 90, 92, 94, 96, 98 are shown for five different positions of the scan deflector 91. FIG. 4 shows a corresponding spot diagram on the image plane 101. Five spots 40, 42, 44, 46, 48 can be seen, as created by the light beams following the light paths 90, 92, 94, 96, 98, respectively. The white circles on top of each spot 40, 42, 44, 46, 48 illustrate the Airy focussed spot diameter of 20,6 μm. It can be seen from FIG. 4 that the actually obtained spot diameter at the image plane 101 is much larger than 20 μm, and furthermore that the spot size over the width of a scan line on the image plane 101 is very variable, the size of the spots 40, 48 at the extremes of the scan line being much larger than 20 μm, the size of the spot 44 at the center of the scan line being larger than 20 μm, but smaller than the size of the spots 40, 48 at the extremes of the scan line, and the size of the spots 42, 46 between the extremes and the center of the scan line being about 20 μm.
FIG. 5 illustrates line bow in function of scan angle. It can be seen that, in this prior art system there is a significant line bow of +25/−20 μm. This is in general considered under par since scanner designers usually strive for a peak to peak line bow smaller than the spot diameter. From FIG. 3 can be seen that the beams 90, 92, 96 and 98 are not impinging perpendicularly on scan surface 101—meaning that the system is only partially telecentric, and not really telecentric as U.S. Pat. No. 5,168,386 would make believe. A system as described in U.S. Pat. No. 5,168,386 can only achieve near telecentricity with a 500 mm focal length lens system, a 200 mm scan width and 20 μm 1/e2 spot diameter size. Using a system with longer focal lengths is not advised, since the size of the scanner aperture must be larger to provide for the same small focussed spot size. A longer distance between the scanning element and the scanning plane also negatively influences the accuracy of the scanner, because all angular position errors of the scanner deflector due to mechanical imperfections and/or noise on the control signal are multiplied by this distance to result in focussed spot size location errors.
As described above, telecentricity is a highly desirable feature in scanner systems for material processing. In general system performance of all scanners increases when using a telecentric f-theta lens. However, telecentricity becomes a requirement when inspecting highly reflective surfaces or when using a co-axial illumination/detection system since these systems require the inspection to be carried out at surface perpendicular conditions.
It should further be noticed that material processing using pulsed laser sources puts very stringent demands on the scanner system. In such a system the light pulses from the laser are emitted at a rate determined by the laser oscillator cavity design. In most cases it is not possible to fire a laser pulse at the exact timing of an external trigger signal. Using external trigger signal results in a timing jitter of about one cavity oscillator period (i.e. 20 ns for a 50 MHz optical oscillator). Moreover all light pulses are emitted synchronously to this base optical oscillator period. For slow scanner (<10 m/sec) systems this is in general not a problem since the scanned spot does not move significantly (only 0.2 μm) within the 20 ns period. However, fast scanner systems (>100 m/sec) exhibit a spot movement of more than 2 μm within the 20 ns period. This results in a spot (or pixel) placement error that becomes a significant portion of the system resolution (10 μm).
As a summary, a high performance pulse laser material processing system requires:                A small (<50 μm) and constant (variation <5%) spot size across the scanned surface.        Constant light/material interaction parameters, like incidence angle, beam profile, beam energy level and beam peak intensity across the scanned surface.        A highly linear relation between deflector angle and spot location; this eases the control of the deflection unit. A constant deflection rate (deg/sec) provides for a constant scanning speed (m/sec) and thus for a constant focussed spot location step using a pulsed laser.        A near perfect straight line scan (line bow free), to provide, in combination with the near perfect linearity, for a simple linear spot location relation versus the deflection angle.        A short optical distance between the deflector and the scanned surface to minimize spot location errors propagated by inevitable deflector scan angle noise and jitter. Preferably, the ratio between scanned width and deflector to scanned surface distance is close to or lower than 1. In practice, this demands maximizing the maximum deflection angle, while lowering the focal length of the F-theta lens, the product of these determining the maximum scan width.        High repetition pulsed lasers (>4 MHz pulse rate) demand fast and wide scanners (>10 m/sec) to process large surfaces.        Very short pulsed (sub nanosecond) lasers require pulse dispersion free optics. The first two requirements are provided by a diffraction limited telecentric scanner system.        
U.S. Pat. No. 6,022,115 describes an optical scan system for measurement of a three dimensional device. The optical system is designed and configured to meet telecentric and f-θ requirements. The system employs primary and secondary mirrors operating in conjunction with a tertiary deflector mounted on a pivot. Light from a light source produces a beam which is deflected off the deflector and the secondary and primary mirrors, respectively. The primary and secondary mirrors may be rotationally symmetric aspheres, whereby the incoming light, the moving scan light and the optical axes of the mirrors are all lying in a same plane. It is a disadvantage of such system that one of the mirrors blocks the light path, such that the scan can only be done in part of a plane. The cost of the mirror is largely defined by its size, hence making a large mirror of which only half may be used is very expensive. Even if this large mirror would be cut into two, thus generating two mirrors for the same price, large machinery is required, which is rather rare.
US2003/0112485 describes a light scanning system which causes a light spot to scan a surface at a constant speed. The light scanning system includes a light source radiating a light bundle, a deflector which deflects the light bundle, a line image imaging optical system which images the light bundle on a deflecting surface of the deflector as a line image, and a scanning/imaging optical system which images the deflected light bundle on the surface as a light spot. The scanning/imaging optical system consists of a first aspheric mirror which is symmetric with respect to an axis of rotation and is disposed on the light inlet side and a second aspheric mirror which is anamorphic and is disposed on the light outlet side.
The use of anamorphic and toriodal mirrors is disencouraged because of the very high manufacturing cost in near diffraction limited systems, larger (>70 mm) optical elements and low volume manufacturing.